Oxygen versus Sulfur Coordination in Cobalt Superoxo Complexes: Spectroscopic Properties, O₂ Binding, and H-Atom Abstraction Reactivity

Abstract

A five-coordinate, disiloxide-ligated cobalt(II) (S = 3/2) complex (1) was prepared as an oxygen-ligated analogue to the previously reported silanedithiolate-ligated CoII(Me3TACN)(S2SiMe2) (J. Am. Chem. Soc., 2019, 141, 3641–3653). The structural and spectroscopic properties of 1 were analyzed by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR), and NMR spectroscopies. The reactivity of 1 with dioxygen was examined, and it was shown to bind O2 reversibly in a range of solvents at low temperatures. A cobalt(III)–superoxo complex, CoIII(O2 · –)(Me3TACN)((OSi2Ph)2O) (2), was generated, and was analyzed by UV–vis, EPR, and resonance Raman spectroscopies. Unlike its sulfur-ligated analogue, complex 2 can thermally release O2 to regenerate 1. Vibrational assignments for selective 18O isotopic labeling of both O2 and disiloxide ligands in 2 are consistent with a 6-coordinate, Co(η1–O2 · –)(“end-on”) complex. Complex 2 reacts with the O–H bond of 4-methoxy-2,2,6,6-tetramethylpiperidin-1-ol (4-MeO-TEMPOH) via H-atom abstraction with a rate of 0.58(2) M–1 s–1 at −105 °C, but it is unable to oxidize phenol substrates. This bracketed reactivity suggests that the O–H bond being formed in the putative CoIII(OOH) product has a relatively weak O–H bond strength (BDFE ∼66–74 kcal mol–1). These thermodynamic and kinetic parameters are similar to those seen for the sulfur-ligated Co(O2)(Me3TACN)(S2SiMe2), indicating that the differences in the electronic structure for O versus S ligation do not have a large impact on H-atom abstraction reactivity.